It has heretofore been proposed to provide a cased glass stream for forming glassware having layered wall segments. U.S. application Ser. Nos. 08/374,371 and 08/374,372 disclose techniques for delivering such a cased glass stream in which core glass from a first source is delivered through a first orifice. A second orifice is vertically spaced beneath and aligned with the first orifice, and is surrounded by an annular chamber that communicates with the second orifice through a gap between the first and second orifices. A heated tube delivers casing glass from a second glass source to the annular chamber that surrounds the second orifice. Glass flows by gravity through the first and second orifices from the first and second sources in such a way that a cased glass stream emerges from the second orifice. This cased glass stream may be sheared by conventional techniques to form individual cased glass gobs for delivery to conventional individual section glassware forming machines.
Although the techniques disclosed in the noted patent applications address and overcome problems theretofore extant in the art, further improvements remain desirable. For example, the presence of air bubbles, sometimes termed "blisters," in the casing glass stream has been a problem. Flow of casing glass from the casing glass spout is controlled by a spout tube, which is positioned over the casing glass spout outlet opening so as to meter casing glass flow at the desired volumetric ratio relative to core glass flow. However, the volumetric ratio of casing glass flow to core glass flow is very low, such as on the order of 5 to 10%. Consequently, when using conventional glassware forming equipment, the extremely low volume of casing glass flowing through the casing glass spout outlet forms a thin fall, around one-quarter inch thick, around the outlet opening and around the upper portion of the heated delivery tube, with the volume within this fall being open. After a period of operation, air bubbles or blisters begin to appear in the casing glass sun. It is believed that a chimney-like effect of the heated air within the interior of the spout outlet opening and the interior of the casing glass delivery control tube creates a pressure differential or gradient between the ambient atmosphere outside of the casing glass spout and the interior within the thin glass fall. It is believed that this pressure gradient promotes migration of air through the refractory material of the casing glass spout, and eventually into the thin glass fall within the spout outlet.
A number of techniques have been proposed in an effort to eliminate this air bubble or blister problem, including lining of the spout outlet opening with platinum in an effort to block air migration. The technique that is currently preferred is periodically to "flood" the casing glass outlet and heated delivery tube with casing glass far in excess of that needed for forming the cased glass stream, and to maintain this excessive casing glass flow for a period of time. It is believed that this "flooding" of the casing glass delivery path eliminates the chimney effect previously described, and further that hydrostatic pressure on the casing glass promotes flow of casing glass into the refractory material of the spout outlet opening so as to block air migration paths. When casing glass flow at reduced level is resumed, the air bubbles or blisters are eliminated for a period of time. However, continued use of the ceramic spout requires that the described "flooding" operation be undertaken with increasing frequency, apparently due to increasing erosion and wear of the spout material. It is believed that, as the refractory spout material ages, it becomes more difficult to fill the air migration cracks and passages with casing glass spout. In any event, the described "flooding" operation detracts from glass production, and therefore undesirably increases production costs. Furthermore, production of cased glass having air bubbles or blisters in the casing layer results in undesirably increased scrap rates, further increasing production costs.
It is therefore a general object of the present invention to provide a method and apparatus for delivering a glass stream, particularly a cased glass stream, in which formation of air bubbles or blisters in the thin glass fall of the casing glass stream is reduced or eliminated, and in which the need periodically to "flood" the glass stream delivery path is also eliminated. Another and related object of the present invention is to provide a method and apparatus for delivering a glass stream, particularly a cased glass stream, that is characterized by improved production efficiency and therefore reduced manufacturing cost as compared with similar prior art techniques.